Do you think El is the monster?

She is not the monster, it was already living on the other side when she showed up but used her to get through. It's because she technically brought it over that she is the only person that can take it out

My bf and I have a theory that Ele and the monster are 2 sides of the same coin, like it's her counterpart in the other world and they are connected. In this sense, one can't exist without the other but they aren't the same being strictly speaking.

There's a lot of interesting theories on Reddit to look at. One of the most interesting was how telekinesis is directly related to plants, etc. With the monster being part plant, almost gene spliced with a human (human body like/plant head), and El being able to tap into these abilities, one of the theories is that they are essentially "brother and sister"